This application claims the priority of Japanese Patent Application No. 8-263477 filed on Sep. 12, 1996 and No. 8-267924 filed on Sep. 17, 1996 which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an all-pixels reading type electronic endoscope apparatus and, more particularly, to an all-pixels reading type electronic endoscope apparatus which reads all the pixels stored by an image sensor and which can adjust the quantity of light irradiated onto the object of observation to an appropriate value.
2. Description of the Related Art
In a CCD (Charge Coupled Device) which is an image sensor of an electronic endoscope, a video signal is obtained by reading an electric charge stored for each pixel by a photoelectric transducer. In a simultaneous electronic endoscope, a color filter for each pixel is disposed on the upper surface of the CCD so as to produce a color picture.
FIG. 10A shows the arrangement of color filters. As shown in FIG. 10A, for example, Mg (magenta), G (green), Cy (cyan) and Ye (yellow) filters are disposed for the respective pixels on the imaging surface of a CCD 1. Therefore, the CCD 1 obtains stored electric charges from the light which passes such color filters.
FIG. 10B shows the signals read by a conventional color difference line-sequential pixel mixture signal reading method. In this method, the stored electric charges of the pixels in a pair of horizontal lines are output in the form of a mixture. For example, at a first exposure, the video signals on lines 0 and 1, the video signals on lines 2 and 3, . . . in the odd (Odd) field are read out in the form of respective pixel mixture signals, and at a second exposure, the video signals on lines 1 and 2, the video signals on lines 3 and 4, . . . in the even (Even) field are read out in the form of respective pixel mixture signals. Therefore, a pixel mixture signal on two lines in the CCD 1 constitutes a signal on one line in a field picture.
FIG. 11 shows the signal reading operation in the CCD 1. An odd field and an even field are formed at intervals of 1/60 sec. (vertical synchronization period) as shown in a field O/E signal (A). Therefore, signals are stored at a storage (exposure) time t of an electron shutter (B), for example, and the signals are read out in the form of a pixel mixture signal in the next period of 1/60 sec.
As a result, (n-1) Odd field signals and n Even field signals are obtained, as shown in the read signal (C). The (n-1) Odd field signals are composed of the pixel mixture signals on lines (0+1), (2+3), (4+5) . . . , which are shown on the left side in FIG. 10(B) and the n Even field signals are composed of the pixel mixture signals on line (1+2), (3+4) . . . , which are shown on the right side in FIG. 10(B). By the interlaced scanning of the Odd field signals and the Even field signals, a picture for one frame is produced and displayed on a monitor.
In the above-described simultaneous electronic endoscope apparatus, however, since there is a time lag of 1/60 sec. when an odd field picture and an even field picture which constitute a picture for one frame are output one after another, if the endoscope itself or the object of observation moves during this 1/60 sec., the picture quality is deteriorated. Especially inconveniently, the picture quality is also deteriorated in a still picture displayed by the operation of a freeze button so as to enable detailed observation of a specified part.
According to the function of the electronic shutter of an electronic endoscope, it is possible to shorten the storage time in a light place so as to improve the picture quality. However, since there is a time lag of 1/60 sec. between the two storage times (exposures) for producing a picture for one frame, as shown in (B) of FIG. 11, the effect of the shortening of the storage time is not always gained.
A light source device of an electronic endoscope is provided with an iris controller for controlling the quantity of irradiated light to an appropriate value. However, when a still picture is displayed, the iris controller does not work for that period. The iris controller generally inputs a luminance signal which is formed from video signals and adjusts the stop so that the luminance signal has a predetermined value. When a still picture is displayed, the quantity of light is controlled on the basis of the luminance signal of the video signals for the still picture.
For this reason, the quantity of light is controlled on the basis of the past data at the point of time when the freeze switch is operated. If the end portion of the endoscope is moved during the display of the still picture and the distance between the end portion of the endoscope and the object of observation is changed, halation is caused or the screen becomes dark when the still picture is changed over to a moving picture. If the quantity of irradiated light is unnecessarily large, the part of observation sometimes gets burned during long-term observation.
In an electronic endoscope apparatus, when a function of adjusting the quantity of light is used, the storage time (shutter speed) is fixed at a predetermined value. On the other hand, when the electronic shutter function is used, the stop is fixed at a predetermined value. However, the function of adjusting the quantity of light is advantageous in that the light output from the light source is made variable, while the electronic shutter function is advantageous in that a sharp picture is obtained with a high shutter speed and in that a good picture quality is obtained even if the endoscope itself or the object of observation moves. Consequently, it is desirable to use the advantages of both functions at the time of taking an image. Especially, in an electronic endoscope device which can display a still picture for the purpose of detailed observation of a specified part, if it is possible to produce a good still picture even if the endoscope itself or the object of observation moves, the endoscope becomes more convenient.